1. Field of the Invention
The present invention relates to an exposure process and more particularly, to an exposure method and an exposure system that are effective for transferring patterns of geometrical shapes on an exposure mask to an optical resist film on a semiconductor wafer in semiconductor device fabrication.
2. Description of the Prior Art
In the exposure process of semiconductor device fabrication, recently, the patterns of geometrical shapes to be transferred have been miniaturized and at the same time, the wavelength of the exposing light has been decreased more and more. As a result, the multiple interference phenomenon of the exposing light has become unable to be ignored.
Specifically, since an optical resist film is formed on an underlying base material (typically, a semiconductor wafer or a conductive/insulating layer), the effective exposure energy of the film varies dependent upon the reflectance of the base material. Also, the effective exposure energy is affected by the optical absorption coefficient of the resist film. Accordingly, the exposure condition needs to be changed according to the combination of the resist film and the base material.
To cope with this need, conventionally, an exposure systems shown in FIG. 1 was developed, which was disclosed in the Japanese Non-Examined Patent Publication No. 4-148527 published in May 1992.
As shown in FIG. 1, this conventional exposure system is equipped with an optical detector 20 provided around a reduction projection lens system. Exposing light is illuminated through the lens system to an optical resist film formed on a semiconductor wafer, and is usually reflected by the wafer surface. The detector 20 detects the reflected light and outputs an electric signal to an integrating circuit 21.
The integrating circuit 21 serves to integrate the output signal during the exposure time, thereby obtaining the total optical energy of the reflected light.
On the other hand, the data of a preset exposure energy is input to a processor 22. The processor 22 converts the input data to the data of the optical energy.
A comparator 23 compares the data of the total optical energy of the actual reflected light sent by the integrating circuit 21 and the data of the optical energy sent by the processor 22. When both of the data accord with each other, the comparator 23 outputs a shutter closing signal to a shutter control circuit 24. The shutter control circuit 24 controls to close a shutter placed in the optical path of the exposure system, thereby stopping the exposure to the resist film.
When each exposure process starts, a start signal is input into the shutter control circuit 24. In response to the start signal, the circuit 24 outputs a reset signal to the integrating circuit 21. The operation of the circuit 21 is reset and restarts by the reset signal.
With the conventional exposure system shown in FIG. 1, the opening (i,e., exposing) time of the shutter can be controlled according to the base material having various reflectance values. Also, the shutter control is effective even when the resist film contains any dye and therefore has a reduced reflectance value.
Generally, the chemical reaction of an optical resist film due to optical exposure is affected by incident exposing light which is directly illuminated and reflected light which is reflected by an underlying base material. The exposed region of the resist film tends to become transparent due to a bleaching phenomenon, resulting in the increased transmittance and the increased intensity of the reflected light. Therefore, the intensity increase of the reflected light due to the optical exposure needs to be considered.
With the conventional exposure system shown in FIG. 1, however, the intensity increase of the reflected light is not considered during the integration process by the integrating circuit 21 and as a result, a problem that the exposure of the resist film is not always optimized occurs.
Further, the thickness of the optical resist film should be kept constant for all semiconductor wafers to be used. However, it is very difficult and consequently, some thickness fluctuation will take place. The thickness fluctuation of the resist film between the wafers causes to change the intensity of the reflected light.
Accordingly, with the conventional exposure system shown in FIG. 1, some error will occur in the exposure energy of the optical resist film. This error is also a cause of the above problem.